In situ combustion process



Sept 12, 1961 w. s. WALLS 2,999,539

IN SITU COMBUSTION PROCESS Filed Nov. '7, 1957 FIG.

IR HOT DUCT GA I6 20 24 P RO OF F HE INVENTOR. FIG. 2 W5. WALLS BYW%%7 A T TORNEVS United States Patent 2,999,539 1N SITU COMBUSTION PROCESS William S. Walls, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Nov. 7, 1957, Ser. No. 694,945 8 Claims. (Cl. 166-39) This invention relates to a process for initiating combustion in a carbonaceous stratum for the purpose of recovering hydrocarbons therefrom.

The recovery of hydrocarbons from carbonaceous strata has been given added impetus in the petroleum industry of late. In this type of process the carbonaceous material in the stratum is ignited by any suitable means and the ignited area is supplied with oxygen either by direct injection of air through the ignition borehole or by inverse injection of air from surrounding boreholes. The combustion in the stratum burns only a portion of the hydrocarbon therein and drives another substantial portion out of the formation into a production borehole from which the hydrocarbons are recovered in conventional manner.

In practicing this type of recovery of hydrocarbons it has been found diificult to establish a combustion Zone of sufficient size to facilitate the easy drive of the combustion zone through the carbonaceous stratum. Unless a relatively large section of the stratum around an ignition borehole is raised in temperature considerably above normal stratum temperature, a great deal of difficulty is experienced in making the combustion zone move through the stratum. This is the problem with which the invention i particularly concerned.

Accordingly, it is an object of the invention to provide a process for recovering hydrocarbons from a permeable carbonaceous stratum. Another object is to provide a process for initiating in situ combustion in a carbonaceous stratum. A further object is to provide a process for preparing a section of carbonaceous stratum around an ignition borehole for in situ combustion on a scale which assures continuation of the combustion after its initiation.

Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

In the investigation of countercurrent in situ combustion in a field test, it was found that one of the most important objectives was to get the combustion started in a large enough area around the ignition borehole to determine if propagation of countercurrent in situ combustion was possible under reservoir conditions. Laboratory tests have been made which demonstrate that the maintaining of higher reservoir temperature facilitates the propagation of countercurrent in situ combustion. Tubes packed with tar sand and maintained at 200 F. burned readily, whereas tar sand in unheated tubes generally failed to support combustion.

A broad aspect of the invention comprises passing a hot gas such as combustion gas, air, or other non-deleterious gas at a temperature in the range of about 225 to 350 F. through a section of stratum adjacent an ignition borehole so as to raise the temperature of the selected section to this range and, while the stratum is at such temperature, igniting the carbonaceous material at the ignition borehole by suitable means, and then feeding air or other free-oxygen-containing gas to the resulting combustion zone through the heated section. The process is best operated by drilling spaced-apart injection boreholes in a ring around the ignition borehole spaced therefrom a distance of several feet, such as 3 to 10 feet. It is feasible to drill the injection boreholes thru the stratum by means of conventional drilling equipment from the ignition borehole so as to reduce the cost of drilling, particularly where the stratum is relatively deep. The hot gas is then injected either through the ignition borehole and out the injection boreholes, or vice versa, so as to heat up the annular section of the stratum adjacent the ignition borehole. A preferred method comprises in ecting heated air through the injection boreholes to the 1gnition borehole through the carbonaceous stratum and when the temperature of the whole section of stratum between the ignition and injection boreholes has been heated to the range of or 200 F. to about 300 F., the carbonaceous material at the ignition borehole is heated up by means of an electric heater, a squib, a gasfired heater, or by any other suitable means, to the ignition temperature of the carbonaceous material, at which time the combustion thereof is initiated because of the flow of air through the stratum into the ignition borehole. It is also feasible to heat the walls of the ignition borehole to combustion supporting temperature while the fiow of gas through the stratum is cut off and then initiating flow of oxygen-containing gas, such as air, from the injection boreholes to the ignition borehole through the heated stratum after the ignition temperature has been reached.

A second series of injection boreholes are drilled in the formation through the carbonaceous stratum in a ring or other suitable pattern radially outside of the first series of injection boreholes and usually at a considerably greater distance therefrom than the space between the ignition borehole and the first series of injection boreholes. After combustion has been initiated around the ignition borehole and the combustion zone has been advanced so that it approaches the first series of injection boreholes, the flow of gas through the first series of boreholes is cut off and combustion supporting gas is injected through the second series of boreholes to feed and advance the combustion zone radially outwardly from the ignition borehole and from the first series of boreholes. During this type of operation the ignition borehole is utilized as a production borehole from which hydrocarbons are recovered by conventional methods. It is feasible to close the ignition borehole after the combustion zone has advanced beyond the first series of boreholes .and recover hydrocarbons from this first series of boreholes.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing of which FIGURE 1 is a plan view of one arrange ment of apparatus for effecting the process of the invention and FIGURE 2 is an elevation through a stratum showing a corresponding arrangement of apparatus.

Referring to FIGURE 1, an ignition or production borehole 10 is surrounded by a first series of injection boreholes 12 and a second series of outer injection boreholes 14. An indirect heat exchanger and furnace 16 is connected with ignition well 10 by means of conduit 18. Inlet conduit 20 supplies the heating gas for the carbonaceous stratum and conduit 22 conducts the heated gas to each of the first series of injection wells 12. Additional conventional equipment, not shown, is utilized in injecting combustion supporting gas through boreholes 14. Conduit 24 is utilized for withdrawing hydrocarbon product from well 10.

Referring to FIGURE 2, wells or boreholes 10, 12, and 14 pass through carbonaceous stratum 26 which is naturally permeable or has been fractured and propped to render the same gas permeable. Boreholes 12; are spaced within about 3 to 10 feet of borehole 10 and boreholes 14 are usually spaced at a substantially greater distance Ehan 10 feet outside of boreholes 12 such as 25 to 2000 eet.

To illustrate one method of operation, air is injected thru conduit 20 into indirect heat exchanger and furnace 16, which is preferably gas fired, and the heated air at a temperature of about 300 F. is injected into stratum 26 thru boreholes 12 and passes thru the stratum to borehole from which it is vented either thru conduit 18 and heat exchanger 16 or thru conduit 24. Injection of hot air is continued until the formation between the injection boreholes and the ignition borehole. is raised to about 250 F At this time, the wall of borehole 10 in stratum 26 is raised to ignition temperature by means of a gasfired heater positioned. on a conduit in borehole 10, not shown, the combustion gas. from the heater and the air from the formation being vented through borehole 10. As the temperature of the carbonaceous material at here hole 10 reaches ignition temperature, the presence of air causes ignition thereof and in situ combustion is automatically established. By venting the hot combustion gas through heat exchanger-r16 during the heating of the borehole with the gas-fired heater, air introduced through conduit is raised to the required temperature without additional heat being supplied from the burner connected to the heat exchanger. After the combustion zone has advanced to the proximity of the first series of injection boreholes 12, flow of air therethrough is cut oif and injection through the second series of boreholes 14- is commenced so that the combustion zone is moved outwardly from boreholes 12.toward boreholes 14. During this phase of the process, it is not necessary to heat the injected air.

During the combustion process, hydrocarbons in the stratum are rendered more fluid and pass through the burned out stratum to borehole 10 from which they are removed by conventional methods. When injection through boreholes 12 is terminated these boreholes are sealed to assure passage of air' through the combustion zone.

The temperature of the section of stratum between injection boreholes 12 and borehole 10 must be limited during preheating to a temperature of about 300 to 350 F., depending upon the character of the stratum being ignited. If higher temperatures are utilized in some formations there is too much conversion of solid and semisolid carbonaceous material to viscous liquid material which plugs the formation. In others heating to higher temperatures drives so much hydrocarbon therefrom that there is insufiicient carbonaceous residue left to adequately sustain combustion.

It is preferred to heat the section of stratum to a temperature above the boiling point of connate water present but not greater than about 275 F. This type of heating improves the section of stratum for in situ combustion, requires a minimum of heating, and makes it possible to readily ignite the stratum and establish a sufiicient- 1y large heat reservoir to assure moving the combustion zone thru the stratum.

I claim:

1. A process for initiating in situ combustion in a permeable carbonaceous stratum comprising passing a gas at a temperature in the range of about 225 to 400 F. thru a section of said stratum between an ignition borehole and at least one first injection borehole therein within several feet .of said ignition borehole so as to raise the temperature of said section to at least the boiling point of connate water and not over 350 F.; while said section is at such temperature, igniting the carbonaceous material adjacent said ignition borehole; and passing a free-oxygen-containing.gas from said at least one first injection borehole to the ignited area to expand the ignited area and advance the resulting combustion zone countercurrently to the flow of said gas.

2. The process of claim 1 wherein the heating gas and oxygen-containing gas are injected thru at least 3 injection boreholes spaced substantially uniformly around said ignition borehole.

3. The process of claim 2 wherein the heating gas and oxygen-containing gas comprise air.

4. The process of claim 1 including the steps of cutting off the flow of oxygen-containing gas thru said at least one first injection borehole as the combustion zone approaches same and feeding oxygen-containing gas to the combustion zone from at least one second injection borehole radially outside of said first injection borehole.

5. A process for initiating combustion in a permeable carbonaceous stratum and recovering hydrocarbons therefrom by in situ combustion comprising passing a gas at a temperature in the range of about 225 to 400 F. radially thru an annular section of said stratum between an ignition borehole and a series of surrounding first injection boreholes spaced from said ignition borehole a distance of several feet so as to heat said section to a temperature of at least the boiling point of connate water and not above 350 F.; While said section is at said temperature, heating the carbonaceous material at said ignition borehole to ignition temperature; passing air thru said section from said first injection boreholes to the hot area at said ignition borehole to initiate combustion of carbonaceous material; continuing air injection thru said first injection boreholes to advance the resulting combustion zone outwardly thereto; closing said first injection boreholes and injecting air into a series of second injection boreholes radially outside of said first series to feed said combustion front; and recovering hydrocarbons produced by the combustion thru said ignition borehole.

6. The process of claim 5 wherein said gas comprises air and heating of the carbonaceous material to ignition temperature is efiected while heated air is flowing to said ignition borehole from said first injection boreholes.

7. The process of claim 5 wherein said gas is injected thru said first injection boreholes and is passed to said ignition borehole to heat said annular section.

8. The process of claim 5 wherein said gas is injected thru said ignition borehole and is passed to said first injection boreholes to heat said annular section.

References Cited in the file of this patent UNITED STATES PATENTS 2,793,696 Morse May 28, 1957 2,818,117 Koch Dec. 31, 1957 2,917,112 Trantham et a1. Dec. 15, 1959 

